As disclosed in U.S. Pat. No. 4,876,492, an electronically commutated motor, ECM, includes a permanent magnet rotor assembly. The rotor assembly has a low reluctance core formed as an annular cylindrical ferromagnetic member generally referred to as backbone. The backiron is surrounded by a number of thin flat angular ferromagnetic elements or laminations secured to the outer peripheral surface of the backiron so as to establish constant magnetic polar regions with north-south polarizations. The thin ferromagnetic elements are made of permanent magnetic material such as Alnico and are secured, as by adhesive bonding, to the backiron. The thin ferromagnetic elements are alternately polarized to provide constant magnetic polar regions.
Where a rotor is to be directly attached to a crankshaft, or the like, it is common practice to heat the rotor and place it on the crankshaft so as to be shrunk fit thereon upon the cooling of the rotor. Ferromagnetic materials, however, are subject to the loss of their magnetic properties when heated to their Currie temperature. As a result, it is conventional to secure the rotors of ECM's by slot and key or other suitable means other than shrink fitting. Where one member is attached to another by shrink fitting, it is conventional to heat one of the members by an external induction coil or to place it in a convection oven to cause its thermal expansion. The heated member is placed on the other member and allowed to cool to a shrink fit. In such a method, the heated member is heated as a unit since the interior bore must be thermally expanded.